1. Field of the Invention
This invention is in the field of radio direction finding systems and methods therefor, and more particularly is an improved radio direction finding system utilizing Doppler frequency shift principles whereby frequency modulation is imparted to the received signal by electronically moving the effective receiving antenna location along a circular path such that the modulation phase angle indicates the bearing of the radio transmission.
2. Description of the Related Art
Radio direction finders (RDFs) are used to indicate the angle of arrival of an incoming radio frequency wave front for the purpose of locating the source of the transmission. A single RDF may be used on a mobile platform to home in on the source of the transmission, or a network of RDFs may be used to locate the transmission source by triangulation.
An excellent type of RDF is that utilizing the synthetic Doppler method. In this system, a circular array of fixed antenna elements is switched sequentially or combined electronically so as to simulate the rotation of a single antenna element in a circle. See for example, U.S. Pat. No. 4,551,727 "Radio Direction Finding System" by David C. Cunningham. As the simulated antenna approaches the direction of the incoming wave front, the apparent frequency increases due to the well known Doppler Effect. Similarly, when the simulated antenna moves in the same direction as the wave front, its apparent frequency decreases. This up-down frequency modulation is recovered by connecting the RDF antenna to a standard frequency modulation receiver and can be heard as a tone at the audio output of the receiver. The frequency of the tone is the same as the rotational rate of the antenna, and the phase of the tone is related to the direction of arrival. The RDF provides the signal processing to measure this phase and display the hearing to the transmission source.
One of the many applications in which RDFs are utilized is to locate vehicles (police cars, delivery vans or stolen cars). In the case of stolen vehicle location, the transmitter is generally activated by the police using radio control after the vehicle is reported missing. See for example U.S. Pat. No. 4,818,998 by Apsell and Stapelfeld. Additional applications include the tracking of shipped packages by freight carriers or others where it is desired to verify the location of the shipped package.
In such applications, it is necessary to distinguish which of many vehicles is transmitting the signal that is being tracked. This is most commonly accomplished by utilizing a pulsed transmission which is frequency modulated by a digitally coded sequence to indicate the identity of the vehicle. A variety of frequency shift keying techniques (FSK, MSK, GMSK, CPFSK, etc.) are used to carry the digitally encoded identification number of the transmitter on the pulsed signal. Sec for example, the MX-COM Products and Applications book which discusses these techniques and presents integrated circuits that are used to encode and decode FSK signals (Available annually directly from MX-COM Inc. of Winston-Salem N.C.)
There are several compatibility problems that arise when a synthetic Doppler RDF is used with an FSK coded pulse transmission. The frequency modulation of the encoded signal interferes with the recovery of the phase information in the RDF due to the overlapping of the FSK modulation with the frequency modulation of the simulated antenna rotation. Furthermore, the frequency modulation due to the antenna rotation interferes with the recovery of the FSK coded signal information. These problems can be alleviated to some extent by judicious selection of the frequency modulation frequencies (FSK tones) and the rotational speed of the RDF antenna. However, this approach is limited by the audio bandpass of the receiver, the modulation sidebands of both the FSK modulation and the RDF antenna modulation, and non-linearities in the receiver.
Another problem is that the signal level required to provide acceptable FSK demodulation is typically several dB higher than that needed to produce a stable bearing display. This can result in the RDF displaying bearings toward transmitters other than the one desired. Such extraneous signals can be due to other transmission sources, intermodulation or harmonics of broadcast signals, and interference.
Therefore, a need existed to provide a radio direction finder utilizing the Doppler principle that is not subject to the bearing errors caused by the FSK modulation. Additionally, a need existed to provide FSK decoding of the transmitted identification data in a Doppler direction finder that is not subject to the errors caused by the simulated rotation of the antenna.